The crystal structure and photoluminescence properties of undoped and Ce3+- or Tb3+-doped Lu-alpha-Sialon are reported. Lu-alpha-Sialon with the composition of Lu0.367Si9.9Al2.1ON15 crystallizes in a trigonal system with a=7.8013(1) A, c=5.6827(1) A, in the space group P31c. The Lu3+ ion is accommodated at the interstice site at 2b and directly connected by seven (N, O) atoms with an average bond length of 2.624 A. The incorporation of large Ce3+ and Tb3+ ions on small Lu3+ site results in the expansion of the unit cell as expected but keeping the average Lu-Ln-(N, O) (Ln=Ce, Tb) distances nearly constant with slight distortion in the lattices. The optical band gap of Lu-alpha-Sialon is about 5.25 eV determined by the diffused reflection spectrum. Lu-alpha-Sialon:Ce3+ (2 mol%) exhibits efficiently greenish-blue emission at about 486 nm under near-ultraviolet (UV) excitation originating from the 5d -> 4f15d0 transition of Ce3+. Lu-alpha-Sialon:Ce3+ is a potential candidate phosphor for white UV-LED applications due to its high quantum efficiency and excellent thermal stability. Lu-alpha-Sialon:Tb3+ (2 mol%) emits strong mixed blue and green light in equivalent due to the transitions of 5D(3)-> 7F(J) and 5D(4)-> 7F(J) of Tb3+.